TW201342990A - Light-emitting diode driving circuit - Google Patents

Abstract

A Light-emitting diode (LED) driving circuit for parallel-driving light bars includes a flyback converter, a buck-boost converter and a current-mirror current-equalizing circuit, in which each light bar includes LEDs. The LED driving circuit uses dual voltage sources having a first positive voltage and a negative voltage to supply an operating voltage of the light bars. In the light bars, the LEDs which operates between the first positive voltage and 0V are directly driven by the high first positive voltage outputted from the flyback converter without using a DC-to-DC converter, and only the LEDs which operates between 0V and the negative voltage are driven by the low negative voltage into which a low second positive voltage outputted from the flyback converter is converted by the buck-boost converter, so that it has high power conversion efficiency. In addition, the elements of the buck-boost converter may use elements having low voltage endurances so as to reduce cost.

Description

Light-emitting diode driving circuit

The present invention relates to a Light-Emitting Diode (LED) driving circuit, and more particularly to an LED driving circuit for driving a plurality of strings in parallel.

1 is a circuit diagram of a conventional LED driving circuit. Referring to FIG. 1, the LED driving circuit 1 includes a flyback converter 11, a boost converter 12, and a current mirror current sharing circuit 13. The LED driving circuit 1 is used for driving a plurality of light strings 31 to 3m in parallel, wherein each light string includes a positive terminal, a negative terminal and a plurality of light emitting diodes coupled in series between the positive terminal and the negative terminal (Light- Emitting Diodes, LEDs) DL1 to DLn, m and n are integers greater than one. This LED driving circuit 1 can be applied to liquid crystal display products such as liquid crystal displays and liquid crystal televisions.

The flyback converter 11 includes a transformer T1, a transistor M1 as a switch, rectifying diodes D1 and D2, capacitors C1 and C2, and a Pulse-Width Modulation (PWM) controller U1. The flyback converter 11 is configured to receive an input voltage Vbus (such as 400V) generated after the rectification filtering and power factor correction of the commercial AC power source, and perform voltage conversion according to the input voltage Vbus to output a first positive voltage Vp1 (such as 24V). ) with a second positive voltage Vp2 (eg 12V). Wherein, the first positive voltage Vp1 is used to further boost the voltage to provide the required operating voltage of the light string 31~3m, and the second positive voltage Vp2 is used to supply the required voltage of the external components such as the timing controller of the liquid crystal panel, and The supply voltage Vcc required for the internal components of the LED drive circuit 1 can be provided.

The boost converter 12 includes an inductor L1, a transistor M2 as a switch, a rectifying diode D3, a capacitor C3 and a PWM controller U2, wherein the PWM controller U2 includes an error amplifier EA and a PWM comparator CMP. The boost converter 12 is coupled to the flyback converter 11 for receiving the first positive voltage Vp1 and performing boost conversion according to the first positive voltage Vp1 to output a third positive voltage Vp3 applied to the light strings 31 to 3m. Positive end.

The current mirror current sharing circuit 13 includes a current mirror 131 having a plurality of outputs and a reference current source 132. The current mirror 131 includes transistors Q11 to Q1m and Q2 and resistors R11 to R1m and R2, wherein the transistors Q11 to Q1m and Q2 are matched, and the resistors R11 to R1m and R2 are matched, and the output of the current mirror 131 is output. (ie, the collector terminals of the transistors Q11 to Q1m) are coupled to the negative ends of the strings 31 to 3m, respectively. Reference current source 132 includes transistor Q3 and resistor R3. When the transistor Q3 is turned on, the supply voltage Vcc generates a reference current Iref through the resistor R3 to be supplied to the current mirror 131, so that the current mirror 131 operates normally and the current flowing through the output terminal thereof is the same, that is, the set of the transistors Q11 to Q1m. The extreme currents I11 to I1m are the same in size, forcing the lamp currents outputted by the negative ends of the lamp strings 31 to 3m to be the same, and achieving the current balance of the lamp strings 31 to 3 m, so that the brightness provided by the lamp strings 31 to 3 m is the same.

In addition, since the collector extreme currents I11 to I1m and I2 of the transistors Q11 to Q1m and Q2 are the same when the current mirror 131 is normally operated, the voltage difference Vr2 between the first end and the second end of the resistor R2 can be detected. The current I2 is about Vr2/R2/(1+1/β), and β is the common emitter current gain of the transistor Q2. Therefore, it corresponds to the detection of the lamp current of the lamp string 31 to 3 m. Then, the feedback current can be controlled by the detected lamp current of the light string 31~3m, so that the lamp current of the light string 31~3m is stabilized and driven to a preset rated current. In this example, the error amplifier EA of the PWM controller U2 is used to compare the difference between the voltage difference Vr2 of the first end and the second end of the coupled resistor R2 and the reference voltage Vref, and the difference is detected correspondingly. The current I2 (corresponding to the lamp current) is different from the rated current. Therefore, the PWM comparator CMP can adjust the ratio of the on and off times of the transistor M2 according to the difference output control signal to adjust the third positive voltage Vp3. The lamp current of the lamp string 31~3m is stabilized and driven close to the rated current.

The existing LED driving circuit 1 is mostly applied to a large-sized liquid crystal display product, and the lamp string 31 to 3 m requires a high operating voltage, for example, up to several hundred volts. In a design, assuming that the reference voltage Vref is 1.25 V and the rated current of each string is 60 mA, the resistance values of the resistors R11 to R1m and R2 are both required to be 20.8 Ω (= 1.25 V / 60 mA). In addition, assuming that the required operating voltage of the lamp strings 31 to 3m is 155V and the collector-to-emitter voltage drop when the transistors Q11 to Q1m are turned on is 3V, the third positive voltage Vp3 needs to be about 160V (≒155V+3V+1.25). V). Since the required operating voltage (such as 155V) of the lamp string 31~3m is all boosted from the low voltage first positive voltage Vp1 (such as 24V) through the boost converter 12 to the high voltage third positive voltage Vp3 (such as 160V). Provided, the boost converter 12 increases power consumption and reduces power conversion efficiency. In addition, the boost converter 12 needs to boost the low voltage first positive voltage Vp1 to the high voltage third positive voltage Vp3, and the voltage stress of the transistor M2, the rectifying diode D3 and the like are high, and the voltage withstand voltage is required. Higher components, so the cost is higher.

In view of this, the object of the present invention is to provide an LED driving circuit which can improve power conversion efficiency and reduce cost.

To achieve the above object or other objects, the present invention provides an LED driving circuit for driving a plurality of strings in parallel, each string comprising a positive end, a negative end and a plurality of series coupled between the positive end and the negative end. LEDs. The LED driving circuit includes a flyback converter, a buck-boost converter, and a current mirror current sharing circuit. The flyback converter receives the input voltage and outputs a first positive voltage and a second positive voltage, and the positive terminal of the string is coupled to the flyback converter to receive the first positive voltage. The buck-boost converter is coupled to the flyback converter to receive a second positive voltage and thereby output a negative voltage. The current mirror current sharing circuit includes a plurality of first transistors, a second transistor, a plurality of first resistors and a second resistor, wherein the first transistor matches the second transistor and each includes the first The first end, the second end and the control end are matched by the first resistor, and the first end and the second end of each of the first transistors are respectively coupled to a negative end of a corresponding string and a corresponding first resistor The first end of the first transistor is coupled to the control end of the second transistor and the first end, the first end of the second transistor further receives a reference current, and the second end of the second transistor is coupled Connected to the first end of the second resistor, the first resistor and the second end of the second resistor are both coupled to the buck-boost converter to receive a negative voltage.

In an embodiment of the invention, the buck-boost converter adjusts the magnitude of the negative voltage according to the magnitude of the string current output by the negative terminal of any of the strings.

In an embodiment of the invention, the buck-boost converter is coupled to the first end and the second end of one of the first resistor and the second resistor. The buck-boost converter adjusts the negative voltage by comparing the difference between the voltage difference between the first end and the second end of the coupled resistor and the reference voltage to adjust the ratio of the on/off time of the buck-boost converter according to the difference .

In an embodiment of the invention, the current mirror current sharing circuit is further coupled to the buck-boost converter, and the reference current is adjusted according to the difference.

In an embodiment of the invention, the current mirror current sharing circuit further includes a third transistor and a third resistor, wherein the third transistor includes a first end, a second end, and a control end, and the first of the third transistor Receiving a supply voltage, the second end of the third transistor is coupled to the first end of the third resistor, and the control end of the third transistor is coupled to the buck-boost converter to receive the difference, the third resistor The second terminal outputs a reference current.

In an embodiment of the invention, the supply voltage is provided by a second positive voltage output by the flyback converter.

In an embodiment of the invention, the first resistor is also matched to the second resistor.

The present invention provides a required operating voltage of a light string by using a positive and negative dual voltage source of a first positive voltage and a negative voltage, wherein the LEDs operating in the first positive voltage to 0V in the light string will be outputted by the flyback converter. The first positive voltage directly provides energy drive, and does not need to pass through a DC-to-DC converter such as a boost converter. Only the LEDs operating in the string of 0V to negative voltage need to be output by the flyback converter. The two positive voltages are converted into low voltage negative voltages by the buck-boost converter to provide energy drive, so the power conversion efficiency is high. Furthermore, since the buck-boost converter converts the low-voltage second positive voltage into a low-voltage negative voltage and the negative voltage is only responsible for providing energy to drive the LEDs of 0V to negative voltage in the string, the transistor and rectifier of the buck-boost converter The voltage stress of components such as diodes is low, and components with lower withstand voltage can be selected, so the cost is lower.

The above and other objects, features and advantages of the present invention will become more <RTIgt;

2 is a circuit diagram of an LED driving circuit in accordance with an embodiment of the present invention. Referring to FIG. 2, the LED driving circuit 2 includes a flyback converter 21, a buck-boost converter 22, and a current mirror current sharing circuit 23. The LED driving circuit 2 is used for driving a plurality of light strings 31 to 3m in parallel, wherein each of the light strings includes a positive terminal, a negative terminal and a plurality of LEDs DL1 DLDL to DLn,m coupled in series between the positive terminal and the negative terminal. n is an integer greater than one. The LED driving circuit 2 can be applied to liquid crystal display products such as liquid crystal displays, liquid crystal televisions, and particularly to large-sized liquid crystal display products.

The flyback converter 21 includes a transformer T1, a transistor M1, rectifying diodes D1 and D2, capacitors C1 and C2, and a PWM controller U1. The flyback converter 21 is configured to receive an input voltage Vbus (such as 400V) generated after the rectification filtering and power factor correction of the commercial AC power source, and perform voltage conversion according to the input voltage Vbus to output a first positive voltage Vp1 (such as 124V). ) with a second positive voltage Vp2 (eg 12V). Wherein, the first positive voltage Vp1 is generally designed to be 80% of the required operating voltage (such as 155V) of the light string 31~3m for direct application to the positive end of the light string 31~3m, and the second positive voltage Vp2 is used for the second positive voltage Vp2 After being converted into a negative voltage Vm, it is applied to the negative ends of the strings 31 to 3m through the current mirror current sharing circuit 23, and the required supply voltage of the external components such as the timing controller of the liquid crystal panel and the internal components of the LED driving circuit 2 are required. Supply voltage Vcc.

The buck-boost converter 22 includes a transistor M2, an inductor L1, a rectifying diode D3, a capacitor C3 and a PWM controller U2, wherein the PWM controller U2 includes an error amplifier EA and a PWM comparator CMP. The buck-boost converter 22 is coupled to the flyback converter 21 for receiving the second positive voltage Vp2 and performing voltage conversion according to the second positive voltage Vp2 to output a negative voltage Vm.

The current mirror current sharing circuit 23 includes a current mirror 231 having a plurality of outputs and a reference current source 232. The current mirror 231 includes first transistors Q11 Q Q1m, a second transistor Q2, first resistors R11 R R1m and a second resistor R2, wherein the first transistors Q11 Q Q1m are matched with the second transistor Q2 and Each includes a first end, a second end, and a control end, and the first resistors R11-R1m are matched with the second resistor R2, and the output ends of the current mirror 231 (ie, the first ends of the first transistors Q11-Q1m) ) is coupled to the negative ends of the strings 31 to 3m, respectively. The first end and the second end of each of the first transistors are respectively coupled to a negative end of a corresponding string and a first end of a corresponding first resistor; for example, the first end of the first transistor Q1i The second end of the first transistor Q1i is coupled to the first end of the first resistor R1i, where i is any integer from 1 to n. The control ends of the first transistors Q11-Q1m are coupled to the control terminal and the first terminal of the second transistor Q2, and the first terminal of the second transistor Q2 further receives the reference current Iref output by the reference current source 232. The second end of the second transistor Q2 is coupled to the first end of the second resistor R2. The first resistors R11 R R1m and the second ends of the second resistor R2 are both coupled to the buck-boost converter 22 to receive the negative voltage Vm.

The reference current source 232 includes a third transistor Q3 and a third resistor R3, wherein the third transistor Q3 includes a first end, a second end, and a control end, and the first end of the third transistor Q3 receives the supply voltage Vcc, The second end of the third transistor Q3 is coupled to the first end of the third resistor R3, and the second end of the third resistor R3 outputs the reference current Iref. When the third transistor Q3 is turned on, the supply voltage Vcc generates a reference current Iref through the third resistor R3 to be supplied to the current mirror 231, so that the current mirror 231 operates normally and the current flowing through the output end thereof is the same, that is, the first power The first end currents I11 to I1m of the crystals Q11 to Q1m are the same in size, forcing the lamp currents outputted by the negative ends of the lamp strings 31 to 3m to be the same, and the current balance of the lamp strings 31 to 3 m is achieved, so that the lamp strings 31 to 3 m are provided. The brightness is the same. In this example, the transistors Q11~Q1m, Q2 and Q3 are both NPN Bipolar Junction Transistors (BJT), so the first end, the second end and the control end are respectively set extremes and shots. Extreme and base extremes.

In addition, since the currents of the first transistor Q11-Q1m and the second transistor Q2 of the second transistor Q1 are the same when the current mirror 231 is working normally, the first end and the second end of the resistor R2 can be detected. The voltage difference Vr2 at the terminal and the current I2 are about Vr2/R2/(1+1/β), and β is the common emitter current gain of the transistor Q2, so it is equivalent to the lamp tube detecting the light string 31~3m. Current size. Then, the feedback current can be controlled by the detected lamp current of the light string 31~3m, so that the lamp current of the light string 31~3m is stabilized and driven to a preset rated current. In this example, the error amplifier EA of the PWM controller U2 of the buck-boost converter 22 obtains a difference by comparing the voltage difference Vr2 between the first end and the second end of the coupled second resistor R2 with the reference voltage Vref. The difference corresponds to the difference between the detected current I2 (corresponding to the lamp current) and the rated current, and the PWM comparator CMP adjusts the ratio of the on and off times of the transistor M2 according to the difference output control signal to adjust the negative The voltage Vm is sized to further stabilize the lamp current of the lamp string 31 to 3 m and drive it close to the rated current. The control terminal of the third transistor Q3 of the reference current source 232 of the current mirror current sharing circuit 23 is further coupled to the error amplifier EA to receive the difference, and the reference current Iref is adjusted according to the difference.

Under the condition that the same current mirror current sharing circuit and the reference voltage are used to drive the same light string with the existing LED driving circuit 1, in a design of the LED driving circuit 2 of the present invention, the reference voltage Vref is 1.25 V per The rated current of a string is 60 mA, the resistance values of resistors R11 to R1m and R2 are both 20.8 Ω, the required operating voltage of the lamp string 31 to 3 m is 155 V, and the collector to emitter extreme voltage when the transistors Q11 to Q1 m are turned on. The voltage is reduced to 3V, and the first positive voltage Vp1 can usually be 80% of the required operating voltage of the lamp string 31~3m, so the first positive voltage Vp1 needs to be 124V (= 155V × 0.8) and the negative voltage Vp3 needs to be about -36V ( ≒124V-155V-3V-1.25V). Since the required operating voltage (such as 155V) of the lamp string 31~3m is provided by the positive and negative dual voltage sources of the first positive voltage Vp1 (such as 124V) and the negative voltage Vm (such as -36V), wherein the light string 31~3m works. The LEDs DL1 DL DLj at the first positive voltage Vp1 to 0V directly drive the high-voltage first positive voltage Vp1 outputted by the flyback converter 21, and do not need to pass through a DC-to-DC converter such as a boost converter. Only the LEDs DL(j+1) to DLn operating at 0V to the negative voltage Vm among the strings 31 to 3m are converted into the low-voltage second positive voltage Vp2 outputted by the flyback converter 21 via the step-up/down converter 22 The low voltage negative voltage Vm provides energy drive, so the power conversion efficiency is high. Furthermore, since the buck-boost converter 22 converts the low-voltage second positive voltage Vp2 into a low-voltage negative voltage Vm and the negative voltage Vm is only responsible for providing energy to drive the LEDs DL DL of the 0V to the negative voltage Vm in the lamp strings 31 to 3m (j) +1) to DLn, therefore, the voltage stress of the transistor M2 of the step-up/down converter 22, the rectifying diode D3, and the like are low, and a component having a low withstand voltage can be selected, so that the cost is low.

While the present invention has been described above in terms of the preferred embodiments thereof, it is not intended to limit the invention, and the present invention may be modified and modified without departing from the spirit and scope of the invention. The scope of protection is subject to the definition of the scope of the patent application.

1, 2. . . LED drive circuit

11, 21. . . Flyback converter

12. . . Boost converter

13,23. . . Current mirror current sharing circuit

131, 231. . . Current mirror

132, 232. . . Reference current source

twenty two. . . Buck-boost converter

31~3m. . . light post

C1~C3. . . Capacitor

CMP. . . PWM comparator

D1~D3. . . Rectifier diode

DL1~DLn. . . LEDs

EA. . . Error amplifier

L1. . . Inductor

M1, M2. . . Transistor

Q11~O1m. . . First transistor

Q2. . . Second transistor

Q3. . . Third transistor

R11~R1m. . . First resistor

R2. . . Second resistor

R3. . . Third resistor

T1. . . transformer

U1, U2. . . PWM controller

I11~I1m, I2. . . Extreme current

Iref. . . Reference current

Vbus. . . Input voltage

Vcc. . . Supply voltage

Vm. . . Negative voltage

Vp1. . . First positive voltage

Vp2. . . Second positive voltage

Vp3. . . Third positive voltage

Vr2. . . Voltage difference across the second resistor

Vref. . . Reference voltage

1 is a circuit diagram of a conventional LED driving circuit.

2 is a circuit diagram of an LED driving circuit in accordance with an embodiment of the present invention.

2. . . LED drive circuit

twenty one. . . Flyback converter

twenty two. . . Buck-boost converter

twenty three. . . Current mirror current sharing circuit

231. . . Current mirror

232. . . Reference current source

31~3m. . . light post

C1~C3. . . Capacitor

CMP. . . PWM comparator

D1~D3. . . Rectifier diode

DL1~DLn. . . LEDs

EA. . . Error amplifier

L1. . . Inductor

M1, M2. . . Transistor

Q11~Q1m. . . First transistor

Q2. . . Second transistor

Q3. . . Third transistor

R11~R1m. . . First resistor

R2. . . Second resistor

R3. . . Third resistor

T1. . . transformer

U1, U2. . . PWM controller

I11~I1m, I2. . . Extreme current

Iref. . . Reference current

Vbus. . . Input voltage

Vcc. . . Supply voltage

Vm. . . Negative voltage

Vp1. . . First positive voltage

Vp2. . . Second positive voltage

Vr2. . . Voltage difference across the second resistor

Vref. . . Reference voltage

Claims (7)

A light-emitting diode driving circuit for driving a plurality of light strings in parallel, each light string comprising a positive end, a negative end and a plurality of light-emitting diodes coupled in series between the positive end and the negative end The LED driving circuit includes:
a flyback converter receives an input voltage and outputs a first positive voltage and a second positive voltage, and the positive ends of the strings are coupled to the flyback converter to receive the first Positive voltage
a buck-boost converter coupled to the flyback converter, receiving the second positive voltage and outputting a negative voltage accordingly;
a current mirror current sharing circuit comprising a plurality of first transistors, a second transistor, a plurality of first resistors and a second resistor, wherein the first transistors match the second transistors And comprising a first end, a second end and a control end, the first resistors are matched, and the first end and the second end of each first transistor are respectively coupled to a corresponding string of lights The first end of the first transistor is coupled to the control end of the second transistor and the first end, and the first end of the second transistor is coupled to the first end of the first transistor Receiving a reference current, the second end of the second transistor is coupled to the first end of the second resistor, and the second resistor and the second end of the second resistor are coupled to the The converter is operated to receive the negative voltage. The illuminating diode driving circuit of claim 1, wherein the buck-boost converter adjusts the negative voltage according to a current of a string current output from a negative terminal of any one of the strings. The illuminating diode drive circuit of claim 2, wherein the buck-boost converter is coupled to the first end of the resistors of the first resistor and the second resistor The buck-boost converter adjusts the difference between the voltage difference between the first end and the second end of the coupled resistor and a reference voltage to adjust the buck-boost converter to be turned on and off according to the difference. The ratio of time is used to adjust the magnitude of the negative voltage. The illuminating diode driving circuit of claim 3, wherein the current mirror averaging circuit is further coupled to the buck-boost converter, and the reference current is adjusted according to the difference. The illuminating diode driving circuit of claim 4, wherein the current mirror current sharing circuit further includes a third transistor and a third resistor, the third transistor including a first end, a second end and a control end, the first end of the third transistor receives a supply voltage, and the second end of the third transistor is coupled to the first end of the third resistor, the third transistor The control terminal is coupled to the buck-boost converter to receive the difference, and the second terminal of the third resistor outputs the reference current. The illuminating diode driving circuit of claim 5, wherein the supply voltage is provided by the second positive voltage output by the flyback converter. The illuminating diode driving circuit of claim 1, wherein the first resistors are further matched with the second resistor.